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# Any math'a maexperts out there? Need help rearranging this formula...

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I have this formula:

How do I work this formula around to equal R2?

I tried a bunch of times but I don't know how to do it I guess...

Thanks!

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hI,

iS THIS WHAT YOU WANTED IT I UNDERSTOOD PROPERLY?

F= 1.44/((R1+2*R2)*C)

(R1+2*R2)= 1.44/F*C

2*R2={1.44/F*C}-R1

R2=[{1.44/F*C}-R1]/2

Sarma

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Thanks! That works... I had F/C instead of F*C some how.... lol

Thanks!

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The datasheet for the LM555 has a graph to select parts for a certain frequency. Changing the resistance of only one of the two frequency-determining resistors affects the duty-cycle of the output.

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The datasheet for the LM555 has a graph to select parts for a certain frequency. Changing the resistance of only one of the two frequency-determining resistors affects the duty-cycle of the output.

Well, I wanted a formula. I made 5 different programs on my calculator so that I can type in values and it will give me an answer. They work so well! It made getting the correct components for my 555 so fast and easy!

Prog1 - CAP:
Give it values:
Frequency: 1.3 Hz
Resistor 1: 1000 Ohms
Resistor 2: 10000 Ohms

It tells me I need a capacitor of ~52.74 uF to make all the values true!

Prog2 - DUTY:
Give it values:
Resistor 1: 1000 Ohms
Resistor 2: 10000 Ohms

It tells me that I have a .09, or 9% duty cycle. I am dealing with a duty cycle of less than 50%...

Prog3 - FREQ:
Give it values:
Capacitor: 52.74 uF
Resistor 1: 1000 Ohms
Resistor 2: 10000 Ohms

It tells me my frequency is ~1.3 Hz.

Prog4 - R2:
Give it values:
Capacitor: 52.74 uF
Frequency: 1.3 Hz
Resistor 1: 1000 Ohms

It tells me that resistor 2 need to be ~10001.44 Ohms.

Prog5 - REVEDUTY
Give it values:
Duty Cycle: .09
Resistor 1: 1000 Ohms
Frequency: 1.3 Hz

It tells me that in order to obtain a 9% Duty cycle, resistor 2 needs to be ~10111.11 ohms and the capacitor needs to be ~52.19 uF.

It is really nice to be able to tell it a few things and it give me an answer rather than trying to figure out the answer your self. Now I need to put all 5 programs into one really nice program and post it online!

I like making programs for my calculator! I've made a few games and even created a program for Ohms law! You type in what you know and it tells you what you don't know. Like, say my current is 10A and my resistance is 60 Ohms, it tells you voltage. It said, "E=I*R:600". It comes in handy on those long days when you don't feel like doing any math!

I then designed a program for the resistor color code. I know it by heart but what the hey. I never got around to finishing it. It gives you a menu of ten colors where you pick the first color. Then another menu of ten to pick the 2nd color, a third for the 3rd color, then a 4th menu to choose the last color, gold or silver. It then tells you the resistance of the resistor and the +- tolerance of it as well.... It's really neat!

NOW YOU KNOW!!!
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There are many programs on the web that calculates parts values for a 555 oscillator.

Maybe your program should allow you to select a standard capacitor value then it has 5% resistor values that are correct for it.

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Ha! Theres a billion and ten ways to do programs...... lol

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